In manufacture or inspection of a functional element product manufactured by micromachining of a surface, such as a semiconductor device and a thin-film magnetic head, a scanning electron microscope (SEM) is widely used for measurement of the width of a pattern formed by machining (referred to as “length measurement” hereinafter), visual inspection and the like. The scanning electron microscope is an apparatus that has an electron source that emits an electron beam, a convergent lens and an objective lens that narrows down the electron beam through interaction between the electron beam and a magnetic or electric field, a deflector that one-dimensionally or two-dimensionally scans a sample with the electron beam, and a detector that detects a secondary signal (a secondary electron, a reflection electron or an electromagnetic wave) emitted from the sample irradiated with the electron beam by making use of the photoelectric effect or the like, and forms a sample image by transforming or processing the detected signal into a visualizable signal, such as a brightness signal, in synchronization with the scanning with the electron beam.
A known approach for an image processing apparatus to perform such imaging is to evaluate a local distortion in the field of view. Patent Literature 1 discloses an approach of measuring an absolute distortion in a measurement standard sample having a periodic structure with a first magnification, imaging a measurement sample having an arbitrary structure with the first magnification and a second magnification (the image taken with the second magnification is used as a comparative image), forming an expanded or contracted image by expanding or contracting the image taken with the first magnification to the second magnification, measuring a relative distortion between the two magnifications in the expanded or contracted image and the comparative image, and determining an absolute distortion for the second magnification from the relative distortion and the absolute distortion described above.
A measuring apparatus incorporating SEM is required to stably perform precise measurement for a long period. In addition, differences in measurements between a plurality of measuring apparatuses in the production line have to be reduced so that the measurements do not vary with the apparatus. To this end, it is essential that the measurement precision of the length measurement electron microscope can be easily monitored, and all the apparatuses in the semiconductor production line are adjusted or calibrated to always perform measurement with an equal precision.
An important factor that determines the measurement precision of the electron microscope is spatial resolution of an incident electron beam. There is a process of measuring edge sharpness for an acquired electron microscope image as an evaluation value of the spatial resolution. Patent Literatures 3, 4 and 5 disclose known processes of measuring edge sharpness for an electron microscopic image, for example.
A known sample used for measuring edge sharpness described in Patent Literature 2 is a conductive organic film with fine particles of a heavy metal, such as gold, deposited thereon. Another known sample is a Si substrate with fine needle-shaped projections of Si formed thereon described in Non Patent Literature 1. Furthermore, as described in Non Patent Literature 2, there is proposed a sample that is produced by dry-etching a Si substrate with a silicon oxide film and a resist deposited thereon to form an irregular pattern of silicon oxide film on the Si substrate. There is a known process of calculating edge sharpness by acquiring an electron microscopic image of such a sample and performing an image processing thereof.
Another factor that determines the measurement precision of the electron microscope is magnification calibration precision of an electron microscopic image. As a conventional calibration sample having a fineness of 100 nm or less in terms of pitch dimension, there is proposed a superlattice sample having a periodic structure described in Patent Literature 6. A known process of calibrating a local magnification of an image with such a magnification calibration sample having a periodic structure is a process described in Patent Literature 1.